1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle having an engine, an electric machine and a multiple-speed, powershift transmission. In particular, the invention pertains to controlling double-step gear shifts in the transmission.
2. Description of the Prior Art
A powershift transmission is a geared mechanism employing two input clutches used to produce multiple gear ratios in forward drive and reverse drive. It transmits power continuously using synchronized clutch-to-clutch shifts.
The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
In a hybrid electric vehicle (HEV) whose powertrain includes a powershift transmission power is supplied from multiple power sources including an engine, and two electric machines: a crank integrated starter/generator (CISG) and an electric rear axle drive (ERAD).
The CISG, which may be located in a drive path between the engine and the transmission input, function as a conventional alternator and starter, and provides torque assist.
The ERAD interfaces with the transmission output and provides torque to the rear driveline for electric-only drive and power boost. The ERAD also provides regenerative recovery of vehicle kinetic energy through the rear driveline while braking the vehicle wheels.
Gear shift between two even-numbered gears or between two odd-numbered gears are referred to a “double step shifts.” When performing double step shifts between gears on the same layshaft, a powershift transmission can produce brief, sharp reductions in power at the transmission input, sometimes referred to a “torque holes.” Torque holes are perceived by the vehicle's occupants as a transient period of harsh shifting.
There is a need in the industry for a technique that reduces or eliminates torque holes when performing double step shifts with a powershift transmission.